1. Field of the Invention
The present invention relates to an image forming apparatus employing an electrophotographic system, such as a printer, copying machine, facsimile and the like.
2. Description of the Related Art
FIG. 13 shows an image forming unit (image producing unit) in a prior art image forming apparatus using the electrophotographic system. The image forming unit comprises a photosensitive drum (image bearing member) 101 having a photosensitive surface layer and being rotatable in a direction shown by an arrow R101, an electricity removing device 102 for removing an electric charge remaining on the photosensitive drum 101 to set a surface charge of the drum to a predetermined uniform charge, a charging unit 103 for uniformly charging the photosensitive drum 101, an optical device 104 for exposing the photosensitive drum 101 to form electrostatic latent images thereon, a developing device 105 for developing the electrostatic latent image, a transferring unit 106 for transferring a toner image on the photosensitive drum 101 to a recording material P or an intermediate transferring body, for example, an intermediate transferring belt, or an intermediate transferring drum, a cleaning device 107 having cleaning blades for removing toner remaining on the photosensitive drum 101 after transferring the toner image, and the like.
The developing device 105 is filled with a two-component developer consisting of a carrier of magnetic powder and toner which is nonmagnetic or weaker in magnetic property than the carrier, the carrier and toner being mixed with each other with a predetermined ratio. A constant developing bias is applied to a sleeve of the developing device 105 which is bearing the toner and carrier to give a predetermined electric charge to the toner. The charged toner is transferred by rotation of a developing sleeve to cause the toner to be deposited onto an electrostatic latent image formed on the photosensitive drum 101, thereby developing the latent image to a toner image.
In processing after completion of an image forming operation, the developing bias of the developing device 105 must be turned off in timing with the removal of the electric charge from the photosensitive drum 101 by the electricity removing device 102. As shown in FIG. 14(a), if the developing bias of the developing device 105 is turned off prior to lowering of the surface electric potential of the photosensitive drum 101 (drum surface electric potential) by the electricity removing device 102, the carrier in the developing device 105 will adhere to the surface of the photosensitive drum 101 because of the potential difference between the surface of the photosensitive drum 101 and the sleeve of developing device 105. As shown in FIG. 14(b), reversely, in a case that the surface potential of the photosensitive drum 101 is lowered prior to the lowering of the potential in the developing device 105, the potential difference between the surface of the photosensitive drum 101 and the sleeve of the developing device 105 becomes the same state as that when normally forming images, with the result that the toner will adhere to the surface of the photosensitive drum 101.
In the case that the toner adheres to the surface of the photosensitive drum 101, although it is a slight amount, toner is uselessly consumed at each end of printing (image forming). Because stoppage of the rotation of the photosensitive drum 101 with the toner remaining thereon is unfavorable, it is required to continue the rotation of the photosensitive drum 101 until the toner remaining on the photosensitive drum 101 has been removed therefrom by the cleaning device 107. On the other hand, when the carrier adheres to the surface of the photosensitive drum 101, there is a risk of the surface of the photosensitive drum being damaged by the carrier in addition to the useless consumption of the carrier. Therefore, such a state should be avoided to the extent possible. According to the prior art as shown in FIG. 14 when the electric potential of the photosensitive drum converges at a lower rate, the timing for removing the electric charge on the photosensitive drum 101 and the timing for turning off the developing bias of the developing device 105 are so determined that the amounts of the carrier and toner adhering to the photosensitive drum 101 are minimized. Moreover, when the potential of the photosensitive drum converges at a faster rate, the timing is so set that the amounts of the carrier and toner adhering to the photosensitive drum become small as shown in FIG. 14(d). In such a case, however, when the rate of convergence varies depending upon the conditions of image forming operations, the contrast of the photosensitive drum potential and developing bias becomes unstable so that the carrier and toner may frequently adhere to the drum. In the case of a lower converging rate of the potential, the contrast of the surface potential and the developing bias becomes partly greater only by taking measures that the surface potential is further lowered and the timing of turning off is shifted, as disclosed in Japanese Patent Application Laid-Open No. 333,687/1993, so that the problems described above would occur. By constructing the device such that attenuation curves of the potential (G) of the photosensitive drum and the developing bias (F) to be applied to the sleeve are caused to be coincident with each other to the extent possible as shown in FIG. 15 and as disclosed in Japanese Patent Application Laid-Open No. 49,375/1987, it may be possible to avoid extreme enlargement of the contrast of the potential of the photosensitive drum and the developing bias, even if the converging rate varies depending upon conditions of image forming operations.
However, using the approach causing the attenuation curve of potential or voltage to be coincident with the attenuation curve of other potential or voltage, the following problems occur. Namely, in the case of curvilinearly varying the potential or voltage to be brought into coincidence, the resolution of the part at which potential or the like is switched over must be elevated. For this purpose, electric circuits must be constructed with higher accuracy and must be complicated for timing sequence. The same effects can be obtained by switching the potential or voltage in a plurality of steps within the range in which no adherence of the carrier and toner occurs without curved attenuating characteristics of potential or voltage.